Manufacture of electron discharge tubes having a photo-conductive target



March 27, 1962 P. SCHAGEN ETAL 3,027,218

MANUFACTURE OF ELECTRON DISCHARGE TUBES HAVING A PHOTO-CONDUCTIVE TARGET Filed Feb. 18, 1959 2 Sheets-Sheet 2 RED ANTIMONY TRISULPHIDE\ 25 7/45, RED ANTIMONY TRISULPHIDE 33 o O CF38 38 o o x O fi X FIG. 5

PIE TER SCHAGEN ANDREW A. TURNBULL.

BY 952M K- AGE Unite This invention relates to the manufacture of electron discharge tubes having a target constituted by a layer of photo-conductive material evaporated onto a supporting base. An example of such a tube is the television camera tube known as the Vidicon.

For a target material such as, for example, red antimony trisulphide, although a satisfactory target layer can be produced by evaporation of the material in a high vacuum from a crucible or like container spaced from the sup porting base, a layer having a porous texture and having a much higher resistivity in the dark is produced when the evaporation is carried out in an atmosphere (preferably of an inert gas such as argon) at a pressure of from a tenth of a millimetre to a few millimetres of mercury. However, the presence of the gas atmosphere considerably reduces the mean free path length of the molecules of the vapourised photo-conductive material as the latter is carried from the crucible to the supporting base by the convection currents that are set up in the gas atmosphere during the evaporation.

It has previously been proposed in Patent No. 2,745,032, issued to S. V. Forgue et al. on May 8, 1956 and entitled Photo-Conductive Targets For Cathode Ray Devices, to carry out the deposition in vacuum or in such a gas atmosphere and after the evaporation to return the envelope of the electron discharge tube to atmospheric pressure and then introduce the electron gun assembly. Thereafter, the evacuating and sealing processes are carried out. However, this method has the disadvantage that the deposited target layer is liable to suffer deleterious effects due to contact with air when the discharge tube envelope is returned to atmospheric pressure.

In a second method of producing a porous target layer in the manufacture of a Vidicon camera tube, in which method contact between the target layer and air is avoided, the tube is provided initially with a tubular side arm extending from the main envelope at right angles thereto near the end face or Window of the tube constituting the supporting base onto which the target material is to be deposited. At a stage when the electrode structure has been assembled, a glass foot or base sealed to the main envelope, the air removed and argon or other inert gas at a low pressure introduced, and with the tube arranged With its end face uppermost and the side arm horizontal, a crucible containing the photo-conductive material located in the side arm is moved into the main envelope to a central position some distance below the collector electrode grid mesh of the tube, which mesh is arranged close to the end face of the tube. The crucible, collector mesh and collector anode in the region of the crucible are then heated and the photo-conductive material, which may be antimony trisulphide, is evaporated from the crucible and passes through the hot mesh to deposit on the end face of the tube. Any photo-conductive material which has deposited on the collector anode or mesh is then re-evaporated or converted into a conductive form by a separate heating operation. The inert gas is then pumped out and the exhaust tube sealed and finally the crucible is States Patent 3,627,218 Patented Mar. 27, 1962 withdrawn into the side arm which is then sealed off close to the main envelope.

in the manufacture of electron discharge tubes having photo-conductive targets it is very important to use for the target a material which is very pure and furthermore to maintain this purity while carrying out the manufacturing steps subsequent to the deposition of the target. Even the gases which are evolved in sealing glass parts of the envelope of a tube can contaminate a target and this is one reason why in the second method above described the electrode structure is fitted in position and the foot or base of the tube sealed to the main envelope before the target layer is formed. However, even in this method apart from being relatively complex due to the use of the side arm, a sealing operation is carried out close to the target after its formation and this may give rise to contamination of the target.

An object of the invention is to provide a simpler method of forming such target layers by evaporation of photo-conductive material either in a vacuum or a gas atmosphere which method does not necessitate the use of a side-arm but which still eliminates contact between the target layer and air.

According to one aspect of the present invention there is provided a method of depositing a target layer of a photo-conductive material on a supporting base within an electron discharge tube envelope by evaporation of a supply of the material, which method comprises the steps of forming the supply on part of the internal structure for the tube, positioning within the tube envelope the said structure, sealing a foot or base to the envelope, evacuating the envelope through an exhaust tube, evaporating the supply of the photo-conductive material to form the target layer on the supporting base, and sealingotf the exhaust tube, which sealing-off operation may be effected either before or after the evaporation of the supply of photo-conductive material. The sealing-off operation is preferably carried out after the evaporation of the photo-conductive material since any gases liberated from the electrode structure of the tube during the evaporation step can be withdrawn through the exhaust tube prior to its sealing.

By internal structure is meant any structure within the tube envelope which is not removed from the tube and remains in the tube after final sealing.

According to a second aspect of the invention therev is provided a method of depositing a target layer of a photo-conductive material onto a supporting base within an electron discharge tube envelope by evaporation of a supply of the material, which method comprises the steps of forming the supply on part of the internal structure for the tube, positioning within the tube envelope the said.

structure, sealing a foot or base to the envelope, withdrawing air from the envelope through an exhaust tube, introducing into the envelope gas at a low pressure, evaporating the supply of the photo-conductive material to form the target layer on the supporting base, evacuat ing the envelope and sealing-off the exhaust tube. The gas introduced into the envelope is preferably an inert gas such as argon.

The photo-conductive material may be evaporated from part of the internal electrode structure of the tube.

In a preferred method in accordance with the invention in which the electrode structure for the tube includes a collector electrode grid mesh which, when in position in the tube envelope, is close to the supporting base on which the target layer is formed, the supply of photoconductive material is formed as a layer on the side of the collector mesh that is to be adjacent the supporting base. The supply of material may be formed by evaporation in a vacuum.

As has been remarked previously, when evaporating photo-conductive material in a gas atmosphere, the material is carried to the supporting base on which the target layer is formed by the convection currents set up in the gas. It is generally difficult, however, to control the heating conditions in such manner that the convection currents set up give rise to a target layer of uniform thickness. This is particularly so when photo-conductive material is evaporated from a crucible or boat beneath the collector grid mesh and has to pass through the mesh as in the above-described second method. However, by forming the supply of photo-conductive material as a layer on the side of the collector mesh adjacent the supporting base the control of the evaporation conditions are very much less critical because the photo-conductive material does not have to pass through the mesh and, furthermore, by reason of the fact that the mesh is close to the supporting base on which the target layer is formed, the production of a target layer of uniform thickness is less dependent on a symmetrical distribution of convection currents. Evaporation of photo-conductive material from the collector mesh also results in a simplification of the evaporation stage compared with the sidearm technique in that in the latter the sequence in which the various parts must be heated is critical whereas in the former method the evaporation is carried out in a single stage by heating the mesh.

The production of a very uniform target layer by the preferred method in accordance with the invention is also facilitated in that the supply of photo-conductive material can be readily formed on the collector mesh in a uniform layer by evaporation in a vacuum.

The methods in accordance with the invention permits a sealing operation to be carried out after the formation of the target, because the exhaust tube, which in practice is remote from the target layer, is integral with the foot or base of the tube. It will be appreciated, therefore, that negligible contamination (if any) of the target layer takes place after its formation due to gases released from glass parts during sealing.

The target layer may be formed, if desired, after degassing the electrode structure of the tube and activating the cathode. In general, the electrode structure will be partially degassed before being positioned within the tube envelope. During degassing operations carried out with the electrode structure within the envelope, either before or after the formation of the target, it will be necessary to ensure that the temperature at which the tube is baked for degassing is less than the vaporisation or sublimation temperature of the photo-conductive material. If degassing is completed after the target layer has been produced, the possibility of vaporisation or sublimation of the target material can also be reduced by cooling the supporting base by, for example, a jet of cold air.

Embodiments of the invention will now be described with reference to the diagrammatic drawings in which FIGURE 1 shows a cross-sectional view of a collector ring and grid mesh assembly of a Vidicon camera tube with the parts of the assembly slightly separated for reasons of clarity,

FIGURE 2 shows a bell-jar in which photo-conductive target material is evaporated onto the collector mesh,

FIGURE 3 shows a diagrammatic cross-sectional view of the tube with the collector mesh and other electrode parts in position,

FIGURE 4 illustrates the manner in which the mesh is heated to evaporate off the photo-conductive material,

FIGURE 5 shows an alternative embodiment of the invention in which photo-conductive target material is evaporated from an annular cup or boat forming part of the collector anode of the tube, and

FIGURES 6A-6B show diagrammatic cross-sectional views of the window of the tube before and after, respectively, the deposition of the photo-conductive target material thereon.

FIGURE 1 shows a view of a collector electrode grid mesh assembly of a Vidicon camera tube with the parts of the assembly slightly separated in the interests of clarity and comprising a very fine metal grid mesh 1 located between a fiat ring 2 and a cylindrical ring 3 having an inward flange 4. The ring 2 and fiange'4 are spot welded at a number of points around the ring thus firmly securing the mesh 1. The mesh 1 is made of copper which is first nickel plated and then rhodium plated. The ring 2 and cylinder 3 are made of coppernickel alloy which is first nickel plated and then rhodium plated. Typical dimensions for the assembly are:

Mm. Outer diameter of ring 2 22 Inner diameter of ring 2 19 Height of ring 3 3 Referring now to FIGURE 2, this figure shows a bell-jar 5 resting on a flat base 6,, a sealing gasket being designated 7. The space within the jar 5 is connected by a pipe 8 to an exhaust pump 9 for evacuating the said space. Resting on the base 6 within the bell-jar 5 is a platinum crucible 16 containing a supply 25" of pure red antimony trisulphide. The crucible 10 is electrically connected to terminals 11 and 12 which pass through the base 6 and are connected by conductors 13 and 14 to a current supply source 15. Extending from the base 6 and within the bell-jar 5 is a support 16 having an arm 17 for holding (by means not shown) the mesh assembly 1-4 symmetrically above the crucible 10 and spaced therefrom at a distance of, for example, about 4.5 inches. To evaporate the antimony trisulphide, the bell-jar 5 is first exhausted to 10- mm. of Hg by pump 9 and the crucible 10 and then heated by current from source 15. By carrying out the evaporation in the high vacuum a very uniform layer can be produced on the mesh 1 and ring 2. The vacuum is then released and the coated ring-mesh assembly fitted into position into the Vidicon camera tube envelope together with the other electrode parts of the tube as shown in FIGURE 3.

In FIGURE 3, reference numeral 18 designates the cylindrical glass envelope of the Vidicon, to which envelope a flat circular glass window 18' is sealed via a fiat metal ring 19 of fernico alloy. Within the envelope 18 is located the cylindrical anode 20 of the tube which extends from the region of the electron gun structure comprising cathode 21, control grid 22 and first anode 23, to near the window 18'. As illustrated in FIG- URE 6A, window 18' comprises a layer of glass 40, the inner surface of which is coated with a transparent conducting film 41 of tin oxide and provides the supporting base for the photo-conductive target to be deposited. The anode 20 is maintained central within the envelope 18 by springs 24 which are welded to the anode 20 and are urged against the inner wall of the envelope 13. Supported by the collector anode 20 is the ringesh assembly 14, the cylindrical ring 3 of which is slid over the end of the anode 20. Layer 25 of antimony trisulphidc produced as has been described with reference to FIG- URE-2 is thus adjacent and close to the window 18.

After the collector anode 20 carrying the ring-mesh assembly 1 to 4, and the electron gun structure have been introduced into the envelope 18, a glass foot or base 26 is sealed to the envelope 18. The envelope is then exhausted via tube 27 and argon at a pressure of a few millimetres of mercury is introduced. Reference numerals 28 designate electrode pins.

FIGURE 4 represents diagrammatically apparatus for evaporating the layer 25 (of FIGURE 3) onto window 18' to form the target layer 25. This figure shows the upper part of the Vidicon envelope 18 surrounded by a cooling jacket 29 containing water 30. The heat necessary for evaporating the layer 25 is provided by an eddycurrent heating coil 31 connected to a source 32 of high frequency current. Closely surrounding the fernico ring 19 is a metal cylinder 33. To evaporate the photo-conductive layer from the mesh 1 to form the target 25 as indicated in FIGURE 6B, current from the source 32 is passed through the coil 31. The cylinder 33 prevents undue heating of the ring 19 by absorbing radiation from the coil 31. Undue heating of the ring 19 is very undesirable since this may damage the seal between the window 18 and envelope 18. The water jacket 30, 31 serves to maintain the window 18' cool during the evaporation process. The coil 31 is positioned so that a region of the anode 20 some distance from the mesh 1 and indicated generally by arrows 34 is heated to bright red heat, parts of the anode on either side of this region being heated to dull red heat. The mesh 1 and ring 2 are heated by the heat radiated from the anode region 34 and also by heat conduction through the anode 20. By reason inter alia of the close proximity of the mesh 1 and window 18', which are usually about 2.5 mm. apart, a very even target layer as indicated in FIGURE 6B can be produced on the window.

After the target 25 has thus been formed the argon is pumped out via exhaust tube 27 which is then finally sealed off close to the foot 26.

Another embodiment of the invention is illustrated in FIGURE 5. In this embodiment the antimony trisulphide is evaporated from an annular cup or boat forming part of the collector anode structure. The antimony trisulphide is evaporated through the collector mesh 1 and onto the window 18' to form the target 25'.

The collector anode structure comprises the main cylindrical part 20 and a cylindrical part 36 one end of which is a sliding fit in the cylindrical ring 3 of the mesh assembly and the other end is inwardly turned up to form the cup 35. The anode parts 20 and 36 are joined by metal band 37 to which they are Welded. A coil 38 is shown for heating the annular cup 35, and its high frequency current source is shown at 39. By heat radiation and conduction from the heated cup 35 the mesh 1 is heated and condensation thereon of antimony trisulphide thereby prevented. Although in this method the supply of antimony trisulphide is spaced some distance from the window 18' it is found that a satisfactorily even, i.e., uniform target layer 25' can be formed on the window 18. Furthermore, even though this method involves evaporating photo-conductive material through the mesh 1, it has been found by experiment that the evaporation conditions are less critical than those of the side-arm technique described above.

In the above description the antimony trisulphide is described and shown deposited on the actual mesh 1 and ring 2. However, it is not necessary to extend the layer onto the ring 2.

What is claimed is:

l. A method of making an electron device comprising a sealed envelope and within the envelope an electrode structure including a layer of a contaminant-sensitive photo-conductive material on a support, comprising the steps of first providing an element of the electrode structure outside of the envelope and forming on said element outside of the envelope a supply of said photo-conductive material, thereafter locating within the envelope in its proper position the electrode structure including the element containing the supply of the photo-conductive material, thereafter closing-01f permanently the envelope from the outside atmosphere, and thereafter heating the said element to vaporize the photo-conductive material thereon and deposit same on the said support.

2. A method of making an electron device comprising a sealed envelope and within the envelope an electrode structure including a layer of a contaminant-sensitive photo-conductive material on a support, comprising the steps of first providing an element of the electrode structure outside of the envelope and forming by vaporization and condensation on said element outside of the envelope a layer of said photo-conductive material, thereafter locating Within the envelope in its proper position the electrode structure including the element containing the layer of the photo-conductive material, thereafter closingoif permanently the envelope from the outside atmosphere, thereafter evacuating the envelope and introducing therein a low pressure gas, thereafter heating the said element to vaporize the photo-conductive material thereon and deposit same on the said support, and thereafter reevacuating the envelope and sealing it off.

3. A method of making a television pick-up device comprising a sealed envelope and within the envelope on its end wall a layer of a contaminant-sensitive photo-conductive material and an electrode structure including a grid mesh adjacent the end wall, comprising the steps of first providing said grid mesh outside of the envelope and forming on said mesh outside of the tube envelope by evaporation and condensation a layer of said photoconductive material, thereafter mounting the grid mesh on the electrode structure and mounting the latter Within the envelope, thereafter closing-01f permanently the envelope to the outside atmosphere, thereafter heating the said grid mesh to vaporize the photo-conductive material therefrom and condense same on the said end wall, and thereafter sealing off the envelope.

4. A method as set forth in claim 3 wherein the photoconductive material is red antimony trisulphide.

5. A method of making a television pick-up device comprising a sealed envelope and within the envelope on its end wall a layer of a contaminant-sensitive photo-conductive material and an electrode structure including a grid mesh adjacent the end wall, comprising the steps of first providing said grid mesh outside of the envelope, forming on one side only of said mesh outside of the tube envelope by evaporation and condensation a layer of said photo-conductive material, thereafter mounting the grid mesh on the electrode structure with the layer facing outward and mounting the electrode structure Within the envelope so that the grid mesh lies adjacent the said end wall, thereafter closing-off permanently the envelope to the outside atmosphere and removing the air therefrom, thereafter heating the said grid mesh to vaporize the photo-conductive material therefrom and condense same on the said end wall, and thereafter evacuating the envelope and sealing it off.

6. A method as set forth in claim 5 wherein the vaporization step takes place in the presence of an inert gas at low pressure within the envelope.

7. A method as set forth in claim 5 wherein the layer is formed on the grid mesh in avacuum.

References Cited in the file of this patent UNITED STATES PATENTS 2,077,961 Smith Apr. 20, 1937 2,404,343 Henderson et al July 16, 1946 2,422,427 Louden June 17, 1947 2,730,643 Gier et al. Jan. 10, 1956 2,745,032 Forgue et a1. Mar. 8, 1956 2,809,087 Yeith Oct. 8, 1957 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,027,218 March 27 1962 Pieter Schagen et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 10, for "Nov. 7 1958" read Feb. 21,

Signed and sealed this 31st day of July 1962.

(SEA L) I Attest:

ERNEST W SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents 

1. A METHOD OF MAKING AN ELECTRON DEVICE COMPRISING A SEALED ENVELOPE AND WITHIN THE ENVELOPE AN ELECTRODE STRUCTURE INCLUDING A LAYER OF A CONTAMINANT-SENSITIVE PHOTO-CONDUCTIVE MATERIAL ON A SUPPORT, COMPRISING THE STEPS OF FIRST PROVIDING AN ELEMENT OF THE ELECTRODE STRUCTURE OUTSIDE OF THE ENVELOPE AN FORMING ON SAID ELEMENT OUTSIDE OF THE ENVELOPE A SUPPLY OF SAID PHOTO-CONDUCTIVE MATERIAL, THEREAFTER LOCATING WITHIN THE ENVELOPE IN ITS PROPER POSITION THE ELECTRODE STRUCTURE INCLUDING THE ELEMENT CONTAINING THE SUPPLY OF THE PHOTO-CONDUCTIVE MATERIAL, THEREAFTER CLOSING-OFF PERMANENTLY THE ENVELOPE FROM THE OUTSIDE ATMOSPHERE, AND THEREAFTER HEATING THE SAID ELEMENT TO VAPORIZE THE PHOTO-CONDUCTIVE 